Many materials have piezoelectric properties. The most well known are aluminium nitride (AlN), zinc oxide (ZnO) or else lead zirconate titanate (PZT). The latter is one of a large family of materials having a particular crystal structure. This is the “perovskite” class of materials. This structure is the reason for the piezoelectric and ferroelectric properties of these “ABO3”-structure-based materials. In the most commonly used materials, lead is frequently found in the A site of the lattice.
An electrical component based on a perovskite material may comprise, as schematically illustrated in FIG. 1, the following multilayer on the surface of a substrate S: a layer CPiezo of material lying being a lower electrode Ei and an upper electrode Es. To grow lead-based materials, it is known to use platinum because the latter promotes growth of the deposited material in its perovskite phase. To promote adherence of platinum to the layer beneath (generally silicon oxide), a tie layer Ca may be used. The most commonly used tie layers are titanium, titanium oxide, tantalum or even strontium titanate.
An upper electrode may be deposited on the active material so as to produce a metal-insulator-metal capacitor. This electrode may be made of platinum, ruthenium, ruthenium oxide, iridium, iridium oxide, tungsten, etc.
This upper electrode may also be structured so as to form interdigitated combs in order to actuate the active material in this plane.
Two main multilayer types are used to produce components depending on the intended application.
Multilayers of the first type are called “bulk” multilayers: in these multilayers all the layers are deposited on the substrate. The substrate remains thick relative to the rest of the layers. In this way, the movement induced by the piezoelectric material is limited and coupled to that of the substrate. This type of multilayer is used when it is desired to produce components that do not have a moveable part, as illustrated in FIG. 1. A tie layer Ca, a lower electrode Ei made of platinum, the layer CPiezo of lead-based perovskite active material, and an upper electrode Es are deposited in succession on the surface of a substrate S.
This type of multilayer may be used to produce tuneable capacitors or else ferroelectric memories (FeRAM). These applications make use of the ferroelectric and dielectric properties of the active material.
When it is desired for the component to have a moveable part, making use of the piezoelectric effect for example, the thickness of the component in the active zone, and more precisely the thickness of the one or more layers located under the active zone, is reduced so as to make it possible for a membrane or a cantilever, for example, to move. A diagram of such a multilayer is illustrated in FIG. 2, which shows the following multilayer on an apertured substrate Sev: an elastic layer Cel, a tie layer Ca, a lower electrode Ei, the active layer CPiezo and an upper electrode Es.
This type of multilayer is used to produce sensors, actuators or else resonators. This multilayer is used in “piezoelectric” applications.
Perovskite materials are complex and may present several phases with various orientations. For example, PZT may crystallize in the tetragonal, rhombohedral or monoclinic phase or in a mixture of these various phases (morphotropic composition). The piezoelectric properties of the PZT depend on its composition and its orientation. This dependence is illustrated in FIG. 3 (“Recent Progress in Materials Issues for Piezoelectric MEMS”, P. Muralt, Journal of the American Society, Vol. 91, No. 5, 2008, pp. 1385-1396.).
The abscissa in FIG. 3 gives the composition, i.e. the Zr:Ti ratio of the material. The ordinate gives the value of the e31,f coefficient, which corresponds to the piezoelectric properties of the thin PZT films. Ti-rich PZT crystallized in the tetragonal phase and Zr-rich PZT crystallized in its rhombohedral phase. For a Zr:Ti ratio near 52:48, the PZT crystallized with a mixture of these two phases and the monoclinic phase (morphotropic phase). Maximized piezoelectric properties were obtained for compositions similar to this one. The curve C3a is for material textured with (111) orientation, the curve C3b is for material textured with (100) orientation. The piezoelectric properties are better for the (100) orientation than for the (111) orientation.
In FIG. 4 (“Preparation of (100)- and (111)-textured Pb(Zr, Ti)O3 Piezoelectric Films and Direct Measurement of Their Piezoelectric Constants”, W. Gong, J.-F. Li, X. Chu, Z. Gui, L. Li, Japanese Journal of Applied Physics, Vol. 42, 2003, pp. 1459-1461.), hysteresis loops of the polarization are drawn as a function of the electric field applied to the material for a given composition (MPB lithium niobate or lithium tantalate with two different orientations). The curve C4a is for material textured with (100) orientation and the curve C4b is for material textured with (111) orientation. The ferroelectric behaviour of the material thus depends on the texture of the material, the (111) orientation having a higher polarizability.
It has been demonstrated that PZT with (111) orientation is the best material for FeRAM memories because this orientation provides, for a given PZT composition, the highest remnant polarization, such as described in “High Temperature Deposition of Pt/TiOx for Bottom Electrodes”, U.S. Pat. No. 6,682,772 B1, 27 Jan. 2004 and illustrated in FIG. 4.
For piezoelectric applications, the best properties are obtained for PZT with (100) orientation. This result was reported by P. Muralt et al. in “Recent Progress in Materials Issues for Piezoelectric MEMS”, P. Muralt, Journal of the American Society, Vol. 91, No. 5, 2008, pp. 1385-1396 and S. Trolier-McKinstry in “Thin Film Piezoelectric for MEMS”, S. Trolier-McKinstry, P. Muralt, Journal of Electroceramics, Vol. 12, 2004, pp. 7-17.
The applicant has observed that the (100) orientation has a better transverse piezoelectric coefficient d31 relative to the (111) orientation.
To control the orientation of PZT, a germination layer may be used. It has been demonstrated by P. Muralt that a thin layer of TiO2 deposited on platinum allows the (111) orientation to be obtained, as described in the article: “Texture control of PbTiO3 and Pb(Zr,Ti)O3 thin films with TiO2 seeding”, P. Muralt, T. Maeder, L. Sagalowicz, S. Hiboux, S. Scalese, D. Naumovic, R. G. Agostino, N. Xanthopoulos, H. J. Mathieu, L. Patthey, E. L. Bullock, Journal of Applied Physics, Vol. 83, No. 7, 1998, pp. 3835-3841.
The same team also demonstrated, and described in the article “Textured, piezoelectric Pb(Zrx,Ti1-x)O3 thin films for MEMS: integration, deposition and properties” N. Ledermann, P. Muralt, J. Baborowski, S. Gentil, K. Mukati, M. Cantoni, A. Seifert, N. Setter, Sensors and Actuators A, Vol. 105, 2003, pp. 162-170, that a thin layer of PbO promotes the (100) orientation.
To explain these differences, lead diffusion is partially controlled by the seeding layer, the stress in the platinum and the component materials of the tie layer when the crystallization initiates. Specifically, it is known that crystallization of this type of layer depends on the interface because the crystallization mechanism is a grain-growth limited mechanism.
Another team (G. Fox, Ramtron) used a first layer with a large excess of lead (30%) and a low-temperature anneal to promote (111) growth, as described in U.S. Pat. No. 6,287,637 B1 “Multi-layer approach for optimizing ferroelectric film performance”.
FIG. 5 (“Effects of adhesion layer (Ti or Zr) and Pt deposition temperature on the properties of PZT thin films deposited by RF magnetron sputtering”, Mardare C. C., Joanni E., Mardare A. I., Fernandes J. R. A., de Sa C. P. M., Tavares P. B., Applied Surface Science, Vol. 243, 2005, pp. 113-124.) shows the results of a study that varied the platinum deposition temperature and the process used to carry out the PZT crystallization anneal (FIG. 5a illustrates the result obtained with a conventional oven and FIG. 5b that obtained with a rapid thermal annealer (RTA)). The crystallization of the PZT is highly dependent on these two parameters. Thus, on platinum deposited at room temperature, if the PZT is annealed in an oven, it crystallizes with (100) orientation, whereas, if it is annealed in an RTA, it crystallizes with (111) orientation. The curves in order of increasing ordinate respectively correspond to Pt deposition temperatures of 25° C., 200° C., 500° C. and 700° C.
FIG. 6 (“Pt-base electrodes and effects on phase formations and electrical properties of high-dielectric thin films”, Lee W.-J., Kim Y.-M., Kim H. G., Thin Solid Films, Vol. 269, 1995, pp. 75-79.) shows the dielectric and ferroelectric properties of PZT as a function of the platinum deposition temperature. The hysteresis loops in FIG. 6a relate to permittivity whereas those in FIG. 6b relate to polarization, these improve as the platinum deposition temperature increases. It would appear then that the optimum temperature, maximizing the dielectric and ferroelectric properties of the PZT, is about 400° C.
It is also known that the deposition temperature of the platinum as the lower electrode may have an impact on the reliability of the active film.
FIG. 7a shows the dependence of PZT polarization switching fatigue on the platinum deposition temperature. The polarization fatigue is less rapid if the platinum is deposited at 500° C. compared to platinum deposited at 25° C. FIG. 7b shows the dependence of the residual strain (also called stress) in the platinum layer, before and after the PZT crystallization anneal, on the platinum deposition temperature. The higher the platinum deposition temperature, the lower the residual strain in the Pt layer will be after the PZT crystallization anneal. Finally, FIG. 7c shows the dependence of the fraction of PZT crystallized with (111) orientation on the platinum deposition temperature. The higher the platinum deposition temperature, the more PZT will be crystallized with (111) orientation. FIGS. 7a, 7b and 7c are taken from “High Temperature Deposition of Pt/TiOx for Bottom Electrodes”, U.S. patent application No. 6,682,772 B1, 27 Jan. 2004.
The strain in the platinum layer acting as a lower electrode is therefore a key parameter in controlling the orientation of the PZT layer grown on top.
It is thus clear from everything that is known to those skilled in the art that the platinum deposition temperature influences the growth of lead-based perovskite layers.
The Applicant has already observed that platinum deposited at 25° C. allows (100)-oriented PZT to be obtained whereas platinum deposited at 450° C. allows (111)-oriented PZT to be obtained. This difference may be related to the diffusion of lead into the lower layers of the multilayer.
Moreover, the (100) orientation is the most favourable for actuators.
The platinum deposition temperature also influences the properties of the perovskite material. It seems that the dielectric and ferroelectric properties are poorer on platinum deposited at 25° C. than on platinum deposited at 450° C. Likewise, depositing the platinum at temperature allows the PZT polarizability fatigue to be limited.
It has been shown in the prior art that depositing platinum at 25° C. allows lead-based perovskite material with (100) orientation to be obtained. It has also been shown that depositing platinum at a low temperature leads to poorer ferroelectric and dielectric properties and to more rapid fatigue of the ferroelectric layer. More generally, it has also been shown that the strain in the platinum varies during the crystallization anneal if the latter is not annealed at the crystallization temperature of the active material. In contrast, if the PZT is deposited directly on platinum annealed at the temperature of the PZT crystallization anneal, the properties of the PZT are not as good, the optimum platinum deposition temperature being about 450° C.
The problem that the present invention seeks to solve is how to obtain the desired orientation for the material without however degrading its properties or increasing its fatigue rate.